1. Field of the Invention
The invention relates to an acrylic pressure sensitive adhesive composition which is excellent in adhesion fixing performance and capable of being removed after the use and a pressure sensitive adhesive tape having a pressure sensitive adhesive layer formed with the pressure sensitive adhesive composition. An acrylic pressure sensitive adhesive composition and a pressure sensitive adhesive tape of the invention can also be used in application in which an object to be adhered is semipermanently fixed, while since after the tape is once adhered to an object to be adhered and kept in the state for a prescribed period, the tape can be simply removed without leaving anything, the pressure sensitive adhesive tape is especially preferable for use in removability application.
2. Description of the Related Art
A pressure sensitive adhesive tape has been used in various kinds of industrial fields such as a home electronics product, automobile and OA equipment as adhesion means good in operability and high in adhesion reliability. In recent years, from the view point of resource conservation, parts that can be recycled used in a such a product have been reused in more of cases after disassembly of the used product. In this case, if parts are adhered to each other with, for example, a both sided pressure sensitive adhesive tape, necessity arises for an operation in which the both sided pressure sensitive adhesive tape attached to the parts are removed. Various kinds of proposals have been made on both sided tapes for which such a removabiity is required.
The contents of the proposals were able to be classified into the three contents as followed:
(1) A pressure sensitive adhesive tape that is strong in a tension strength of a support base material thereof and thereby can be peeled off when being removed without breaking the support base material thereof is provided.
(2) A pressure sensitive adhesive tape that is excellent in removabiliy by controlling bulk physical properties of a pressure sensitive adhesive is provided.
(3) A pressure sensitive adhesive tape that is excellent in removability by mixing a specific tackifier resin into a specific acrylic polymer is provided.
Contents of the art that have been proposed are such that almost one, or two or more of the proposals in combination are adopted and thereby a pressure sensitive adhesive tape is provided with excellent pressure sensitive adhesive characteristics, and can be peeled off without being broken when peeled off again with no remains of the adhesive on surfaces of the parts. As for the proposal (1), it is very clear that with a stronger support base material, a pressure sensitive adhesive tape is harder to be broken. With only the use of the proposal (1), however, it is definite that the proposal cannot achieve a task that a pressure sensitive adhesive tape is excellent in pressure sensitive adhesive characteristics and that no adhesive remains are left behind when peeled off again. Therefore, many combinations of the proposals (2) and/or (3) to (1) have been designed.
For example, a proposal has been made on an acrylic pressure sensitive adhesive made of a specific acrylic copolymer and a specific acrylic tackifier resin (oligomer) (see JP-A No. 2003-1930006). A pressure sensitive adhesive layer described in JP-A No. 2003-1930006 has a purpose to prevent degradation in the layer by ultraviolet outdoors and since the pressure sensitive adhesive layer is formed by polymerization with active energy rays, an acrylic oligomer is used as a tackifier resin. However, an acrylic oligomer has a task to be unachieved in pressure sensitive adhesive performance such as repulsion resistance to a non-polar object to be adhered including polyolefin, a pressure sensitive adhesive strength and a constant load peelability. Also, it has been well known that a rosin-based resin improves an adhesiveness to a non-polar object to be adhered as a tackifier resin conventionally mixed into an acrylic pressure sensitive adhesive.
A proposal has been made on a removable both-sided tape using a non-woven fabric defined as to a weight, a grain ratio, a tensile strength, a density and compositions as a support and using a pressure sensitive adhesive having bulk physical properties such as a specific storage elastic modulus and loss tangent (see JP-A No. 2003-253228). A pressure sensitive adhesive tape described in JP-A No. 2003-253228 has purposes to decrease adhesive remains and to suppress breakage of a support base material. However, a task of adhesive remains cannot be solved only by defining the bulk physical properties. Many ideas have been conceived on combinations of acrylic polymers and tackifier resins in order to achieve similar bulk physical properties. With different materials, a different interaction inevitably entails at an interface between a pressure sensitive adhesive and an object to be adhered. That is, the difference of an interaction at an interface between a pressure sensitive adhesive and an object to be adhered varies the pressure sensitive adhesive performance and the adhesive by the selection of the materials. Bulk physical properties are only parameters. Therefore, JP-A No. 2003-253228 cannot be said that the publication discloses conditions that always can achieve compatibility between pressure sensitive performance and the removability.
A proposal has been made on a pressure sensitive adhesive tape having a pressure sensitive adhesive layer with a gel fraction of 60% or more prepared by adding a tackifier resin having a phenol group or an alkylphenol group in a molecule to a specific acrylic copolymer (see JP-A No. 2001-240817). However, in a case where a gel fraction in a pressure sensitive adhesive layer is as large as 60% or more as stated in JP-A No. 2001-240817, pressure sensitive adhesive characteristics, especially, a constant load peelability and a repulsion resistance, are extremely degraded. Therefore, JP-A No. 2001-240817 has been left with a task unachieved with respect to pressure sensitive adhesive characteristics.
A proposal has been made on a removable tape using a pressure sensitive adhesive having bulk physical properties such as a specific storage elastic modulus and a specific loss tangent, obtained by mixing 10 to 40 parts by weight of a tackifier resin to 100 parts by weight of a specific acrylic copolymer (see JP-A NO. 2000-239632). In JP-A NO. 2000-239632, it is described that at least one kind of a polymerized rosin ester resin is contained as a tackifier resin. Adhesive remains are largely affected by an interaction at an interface between a pressure sensitive adhesive and an object to be adhered as described above. Since a tackifier resin is a low molecular weight component, the resin largely affects an interaction at an interface between a pressure sensitive adhesive and an object to be adhered. In JP-A No. 2000-239632, however, there is shown no example of a tackifier resin other than polymerized rosin ester. A pressure sensitive adhesive performance and adhesive remains are largely altered by a kind of a resin used together with a polymerized rosin ester. In a case where a polymerized rosin ester alone is used, if a mixing quantity thereof is small, a pressure sensitive adhesive performance is poor, while, if a mixing quantity thereof is large, a pressure sensitive adhesive force to a non-polar object to be adhered such as a polyolefin is excessively stronger, leading to adhesive remains. Therefore, in JP-A No. 2000-239632, there has remained unsolved a task of compatibility between a pressure sensitive adhesive performance and removability.
A disclosure has been made on a pressure sensitive adhesive obtained by mixing a total quantity of 5 to 45 parts by weight of a thermoplastic xylene resin and a polymerized rosin ester as tackifier resins at a ratio of (1 to 6) to (6 to 1) to 100 parts by weight of a specific acrylic copolymer (see JP-A No. 11-269439). An excessive use of thermoplastic xylene resin as a tackifier resin, however, makes worse a pressure sensitive adhesive performance in a low speed region such as a constant load peelability and a repulsion resistance, which is important practically. In a system having much of a polymerized rosin ester, a pressure sensitive adhesive force to a non-polar object to be adhered such as polyolefin is excessive stronger, resulting in adhesive remains. Therefore, in JP-A No. 11-269439, no compatibility is established between a pressure sensitive adhesive performance and removability, a task has been left unsolved.
No conventional pressure sensitive adhesive, as shown above, satisfies compatibility between performances of “being excellent in pressure sensitive adhesive characteristics and no adhesive remains being left when peeled off again” that conflict with each other, wherein the performances that conflict with each other are not explained logically. To be short, published matters shown above only defines in some form a composition of a polymer of a completed pressure sensitive adhesive tape, and mixing of a tackifier resin and, furthermore, only defines bulk physical properties as parameters. Since the contents of the proposal are not logical, there has been unachieved a task to achieve compatibility between performances of “being excellent in pressure sensitive adhesive characteristics and no adhesive remains being left when peeled off again” that conflict with each other at a high level.